1. Field of the Invention
The present invention relates to an active pixel sensor array, and more particularly to an active pixel sensor array which can reduce the number of elements and the size of capacitors by enabling a reset switching transistor to include a function of an optical sensor, resetting a pixel voltage to a power supply voltage VDD after outputting a gate selection signal, and resetting the pixel voltage to be lower than the power supply voltage due to a coupling function when the gate selection signal is outputted.
2. Description of the Prior Art
Recently, various kinds of image sensors including a conventional charge coupled device (CCD) have been developed and commercialized. As representative image sensor, there are a complementary metal oxide silicon (CMOS) image sensor and an image sensor using an amorphous silicon thin-film transistor (TFT).
In order to convert a strength change of a light for each pixel into an electric signal, The image sensor includes a gate driving circuit for selecting a gate, an optical sensor array 1 composed of pixels 2 for converting the strength of the light into a quantity of electric charge, and a column driving circuit 3 for outputting a voltage value obtained by amplifying the quantity of electric charge generated from the optical sensor array 1, as shown in FIG. 1.
A device for obtaining a picture from the image sensor is divided into an active pixel image sensor for amplifying and outputting a change of the quantity of electric charge generated from an optical sensor in which a resistance value is changed according to the strength of the light for a single pixel, and a passive pixel image sensor for outputting the quantity of electric charge through a switching element directly.
First, with reference to FIGS. 2a and 2b, the passive pixel sensor will be now explained.
An electric current generated from an optical sensor due to light is stored in a storage capacitor (C1) during a frame, and is transmitted to a column driving circuit via a data bus line, if a selection switching transistor (T3) is selected. In this case, a signal voltage outputted by dividing the electric charge between a parasitic capacity of the data bus line and the storage capacitor (C1) within the pixel is decreased. This causes a signal-to-noise ratio to decrease.
Accordingly, it is necessary for a high capacity of storage capacitor to reduce or remove the phenomenon that the signal-to-noise ratio is decreased.
However, since a large size is required in case that the high capacity of storage capacitor is used, there is a problem that the storage capacitor isn't proper to a high-resolution image sensor.
In order to solve the above problem, an active pixel sensor for outputting a voltage obtained by amplifying a quantity of signal charge has been developed and used in recent years.
The active pixel image sensor will be explained with reference to FIGS. 3a and 3b. 
As illustrated in FIGS. 3a and 3b, the active pixel image sensor for converting the strength of the light into the quantity of electric charge and then amplifying the quantity of electric charge to output a voltage, produces a leak of the electric charge stored in the storage capacitor C1 due to an optical current that is produced by the light in optical sensor, to thereby lower a fixed voltage. The voltage is connected to a gate electrode of a source follower transistor (T2), to lower and outputs a difference voltage between a threshold voltage and a gate voltage in case that the selection switching transistor (T3) is selected. The difference voltage is transferred to a column driving circuit through a data bus line. If a reset switching transistor (T1) is selected, the voltage as much as the power supply voltage VDD is stored in the storage capacitor C1, thereby resetting the pixel voltage.
In other words, as illustrated in FIG. 4, according as a n-th gate line of a m-th frame is selected depending on a first gate line selection signal 5 and the selection switching transistor (T3) is turned on, a voltage as much as a difference between a pixel voltage (V1) and a threshold voltage of the source follower transistor (T2) is transferred to the column driving circuit 4 via the data bus line.
Further, while a first reset signal 6 is outputted, a n-th pixel reset switching transistor (T2) is turned on and a pixel voltage is reset to a power supply voltage VDD.
Meanwhile, the pixel voltage is reduced by generating the optical current due to a photo diode (P1) for a signal accumulation time 7 just before the first reset signal 6 is outputted and a n-th gate selection signal of a m+1-th frame is outputted. The photo diode (P1) may be an optical sensor.
Further, a process is repeatedly performed for selecting a n-th gate line of a m+1-th frame corresponding to a second gate line selection signal, turning on the selected switching transistor T3, and transferring a voltage as much as the difference between the pixel voltage (V1) and a threshold voltage of the source follower transistor (T2) to the column driving circuit 4 through the data bus line.
Further, a process is repeatedly performed for resetting the pixel voltage (V1) to the power supply voltage VDD by turning on the n-th pixel reset switching transistor T2) of the m+1-th frame while a second reset signal 9 is outputted, and simultaneously selecting the n+1-th gate line.
However, since the active pixel image sensor, since three transistors, one capacitor, one optical sensor, two selection lines, and two power supplies are required, a high integration of the active pixel image sensor is reduced.